The present invention relates to a code multiplexing communication system used for a wireless local area network (LAN).
Wireless LANs are much more flexible than conventional wire LANs in terms of the locations and movement of terminals. However, since frequencies used for a wireless LAN are limited, it is difficult to increase the number of communication lines in accordance with the number of users as in a wire LAN. A 2.4-GHz ISM (Industrial, Scientific and Medical use) band is one of the frequency bands that the wireless LAN can use. This band is also used for RF heating and the like. For this reason, in order to share the ISM band, it is mandatory to use a spectrum spread scheme in the wireless LANs. In addition, since the bandwidth is limited to 26 MHz in Japan, the transmission rate limit in the conventional wireless LANs is 2 Mbps. At a transmission rate of 2 Mbps, however, the transmission rate per user decreases with an increase in the number of users.
FIG. 2 shows a conventional modulation amplification scheme. FIG. 2 shows the arrangement of a wireless LAN based on the scheme recommended by IEEE 802.11.
The signal transmission rate is 2 Mbps, and a 11-bit Barker code is used as a spreading code for spectrum spread. A spreading code is output from a spreading code generator 203 at a chip rate of 11 MHz. As shown in FIG. 3, a 11-bit Barker code has an auto-correlation value of +1. The correlation value of a Barker code having undergone a timing shift is -1/11 or 0.
One-bit serial data input through a terminal 200 of a transmission apparatus 220 is converted into 2-bit parallel data by a serial/parallel (S/P) converter 201. A spreading unit 202 multiplies the output signal from the S/P converter 201 by a spreading code output from the spreading code generator 203. The output signal from the spreading unit 202 is modulated and converted into an RF signal by an analog transmission processor 204. The RF signal is then output to the air through the antenna.
In a reception apparatus 230, the signal received through the antenna is amplified and converted into a baseband signal by an analog reception processor 205. A correlator 206 calculates the correlation between the baseband signal and the Barker code. As shown in FIG. 4, the correlator 206 is constituted by cascaded delay elements 401 each having a delay corresponding to a chip rate, multipliers 402 each serving to multiply an output from a corresponding one of the delay elements 401 by .+-.1, and adders 403 each serving to add an output from a corresponding one of the multipliers 402, an input signal, and a sum output from the preceding multiplier 402. The multipliers "+1" and "-1" are identical to a Barker code series.
In the correlator 206, a delayed wave caused by multipath reflection in a transmission path is decomposed in units of chips upon spectrum spread, as shown in FIG. 5. Only the chip signal having the maximum correlation value is extracted, but the remaining chip signals produced by the delayed wave are discarded, thereby reducing the distortion caused by the delayed wave. The output signal from the correlator 206 is demodulated by a demodulator 207, and converted into 2-Mbps serial data by a P/S converter 208. The serial data is then output to an output terminal 209.
Means for increasing the transmission rate on the basis of conventional techniques include a method of increasing the symbol rate and a method of increasing the number of bits per symbol.
In the method of increasing the symbol rate, a spreading ratio must be 10 or more, and the band width is limited to 26 MHz in Japan. Owing to such limitations, the transmission rate cannot be expected to be doubled or more.
The method of increasing the number of bits per symbol includes a method using a multiple-PSK or QAM. However, since the signal quality greatly deteriorates due to the influences of delay distortions and the like, and a high arithmetic processing precision is required, it is technically difficult to use such a method. For example, in order to obtain a transmission rate twice that of QPSK, signal multiplexing higher in degree than that of hexadecimal QAM is required. Hexadecimal QAM requires power 10 times that required by QPSK, and is more susceptible to amplification distortions. For this reason, in order to obtain the performance equivalent to that of QPSK, the loads on devices need to be considerably increased. In addition, this method becomes susceptible to the influences of distortions in transmission paths, resulting in a smaller service area.